In a synchronous telecommunications network or system, such as a Synchronous Digital Hierarchy (SDH) or SONET transmission network, traffic data is transported across the network via a transmission path. A path may comprise a number of sections, each section comprising a transport service between two adjacent nodes of the network. Each network node typically comprises a synchronous transmission apparatus, such as a multiplexer, a regenerator or a cross-connect, for receiving the traffic and, where applicable, processing the traffic and transmitting it on to another network node or to a local terminal.
It is common to provide path protection such that the traffic arrives at a target node via two independent paths namely, a main path and a reserve path. If, for any reason, the main path should be compromised or should fail, then the reserve path is selected. Thus, a synchronous transmission apparatus is provided with a respective data port for each incoming path and a single data port for output of the selected path. The transmission quality of each path is evaluated at the respective ingress data port and the results are communicated to the output data port. A quality comparison is made at the output data port and the path of higher quality is selected.
It is increasingly important for telecommunications network operators to be able to provide information to customers concerning the performance of their networks. Typically, performance monitoring involves measuring the quality of the traffic at selected nodes of a given path by performing, for example, parity checks. Where sub-network connection protection is employed, only the performance information for the selected path may be required. This is problematic since the performance of the traffic on the main and reserve paths is normally monitored at the respective ingress data ports while the selection between main and reserve paths is performed at the output data port. It is necessary therefore to communicate, or signal, to each ingress data port whether or not its respective path is selected at the output data port. The ingress data port of the selected path duly compiles the required performance monitoring information and reports the compiled information on to, for example, a central monitoring unit for further processing, or logs the information, as required. The ingress data port of the non-selected path may be arranged to refrain from reporting or logging the information, as required.
It will be appreciated from the foregoing that there is a need for message signalling within a network node and particularly between data ports of a synchronous transmission apparatus.
Normally, each network node includes a microprocessor-based node controller which controls the configuration of the node, including the data ports. It is possible to arrange for the node controller to communicate to the ingress data ports, using a suitable software routine, which path is selected at the output data port. However, such a solution is considered to be too slow, and important monitoring information may be lost by the time the appropriate ingress data port is notified of its path's selection. Alternatively, additional hardware may be provided at the output data port to re-measure or re-calculate the performance data. However, this option involves duplication of circuitry and is undesirable for reasons of cost and size.